Question about stepper vs servo torque rating in regards to X2 cnc mill

[davesharps]

Hey there,
I Just got a cnc fusion kit to finally convert my lms 3990 to cnc, i've been browsing around and have found a lot of people are recommending the nema 23 381oz in steppers off automation technology (AT), however I have been seduced by the silky smooth and quiet action of the clearpath servo motors (budget is pretty liberal) but i'm a bit confused by the torque reading and just want a little clarity.

When the AT stepper is rated at 381 oz in is that just for peak?
Should I be comparing peak to peak?
Does continuous torque play a factor in this comparison?

At the end of the day i'm just wondering if the clearpath cpm-sdsk-231s-rqn is a suitable motor for use on an X2 mill or if it's over/underpowered

Thanks a ton!

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[awerby]

It's more complicated than that. The torque ratings on the steppers are "holding torque" measured at standstill. To find out the torque at a given speed you need to consult a chart that should come with the motor specs. It will be lower than the holding torque, usually considerably less as the speed increases. Many servo motors will have peak torque at a certain speed, with less at very slow or very fast speeds. These need a speed reducer to get into the "sweet spot" of best performance. But looking at the torque curve of the particular one you're contemplating https://www.teknic.com/model-info/CPM-SDSK-2311S-RQN/, it seems more like a stepper in that regard, with torque steadily falling off as speed increases. You should be able to couple it directly to your screws, but whether the baseline ("continuous") torque is sufficient depends on the mechanics of your machine.

[Al_The_Man]

If you look at the manufacturers curves for DC and BLDC servo motors, you will see that maximum torque is typically at zero RPM and is fairly flat up to the maximum rpm.
With Steppers motors, the maximum torque slopes down fairly drastically from zero after a few hundred rpm, modern drives attempt to keep the torque fairly flat for as long as possible.
The continuous torque of servo motor can be exceeded for very brief periods only, otherwise failure can result from overheating,
Al.,

[SCzEngrgGroup]

I have to wonder what you expect to gain using servos, other than wasting a lot of money. Properly-sized, properly-operated steppers will easily deliver more performance and accuracy than an X2 can take advantage of. Why not spend that extra money on a better base machine, than on motors and drivers that will have absolutely NO functional advantage?

Regards,
Ray L.

[109jb]

I agree. Steppers should be more than sufficient for an X2. Anything more and I would be looking at upgrading the machine first.